Microwave slot coupling



Jan. 2o, 19534 J. E. GIBSON 2,626,316

MICROWAVE SLOT COUPLING Filed OQt. 25, 1945 2 SHEETS-SHEET l me/whewJOHN E. GIBSON Patented Jan. 20, 10953 UNITED STATES PATENT yOFFICE(Granted under Title a5, U. s. code (1952), sec. 266) Claims.

This invention relates in general to the transfer of electrical energyand in particular to an arrangement for the coupling of high frequencyenergy between components of a microwave system or between two suchsystems.

In the usual high frequency system, such as is encountered in the fieldof radio echo ranging and related activities, the problem oftransferring energy from one component to another must frequently besolved by means which take into account the fact that even the smallestcircuit element, if possessed of any power-handling ability, hasphysical dimensions one or more of which constitutes an appreciablefraction of a wavelength. This electrical-length property denies asolution by ordinary direct current or low-frequency methods, sincecircuits at very short wavelengths rarely satisfy the conditions whichwarrant the assumption of an infinite characteristie electromagneticvelocity. Just as the need for the satisfaction of these conditionsorfor methods particularly adapted to the problems presented dictatedthe adoption of concentric transmission lines and waveguides, so alsodid they dictate the adoption of energy coupling methods which bearlittle physical resemblance to D. C. or low-frequency coupling methods.

It is an object of this invention to provide a means of transferringenergy at frequencies in the frecquency spectrum above 1000 megacycles'per second.

Itis another object of this invention to provide an efficient,convenient means of transferring energy from one transmission line to asecond transmission line.

It is another object of this invention to provide a coupling-slot meansof transferring energy from one system to another which permits theefficiency of transfer to be maintained at its optimum value over a widerange of frequencies by simple mechanical tuning.

Other objects and features of this invention I will become apparent upona careful consideration of the following detailed description when takentogether with the accompanying drawings in which:

Fig. 1 is a diagram illustrating certain of the principles upon whichthis invention is based;

Fig. 2 is an elevational view of a high frequency generator utilizingone embodiment of this invention 3 is an end view of the embodiment ofFig. 2 partially in cross-section looking in the direc-v tion of arrows3-3 of Fig. 2.

Fig. 4 is a detailed view of the embodiment of this inventionillustrated in Fig. 2.

Reference is now had in particular to Fig. l wherein there is shown adiagram illustrating certain of the principles upon which the teachingsofv this invention are based. Surface I represents a portion of onesurface of a conducting sheet serving to separate, and to act asconducting boundary surfaces for..regions in which electromagneticfields may be caused to exist. At frequenciescorresponding to centimeteror decimeter wavelengths the depth of penetration of the electromagnetickfields intoA the surfacesv of,

the conducting sheet isso small that essentially no mutual interactionbetween the electromagnetic elds is possible even though the conductingsheet serves as a common boundary conductor. Consider an electromagneticfield to be gen-V erated, by means not shown,l in the region bounded bysurface I with such an orientation as to tend to produce a current ow onsurface I everywhere parallel to bounding edges-2 and 3. An aperture 4having a width wwhich is a fractional part of a quarter wavelength and alength.

l equal tol one half wavelength is cut in the conducting sheet with itslong dimension lying perpendicular to the direction inwhich currentktends to ow on surface I. Such an aperture or slot has many propertiessimilar to those of a half-wave two-wire Lecher frame. Consideredlindependently of any shunt impedances provided,- by any circuit systemof which the conducting.

the midpoint and the ends in a manner, similar1 to that obtained with aLecher frame of corre,- sponding length. Since the slot is Jcommon toboth surfaces of the conducting sheet,.it represents a common impedancebetween any circuit.

system of which surface I is a part and any cir:

cuit system of which the oppositey surface is a part. Thus, theprovision of` an electromagnetic eld tending to induce a current insurface I parallel to edges 2 and 3 will tend to cause a current flowinthe opposite surface and thereby a current flow in any circuit system ofwhich this surface is a part.k Further, the provision of anlelectromagnetic field tending to induce a current in surface I notparallel to edges 2 and 3 but of, an orientation suchv that aperture 4has atleast a component of its length dimension perpendicular to thedirection of induced current now will also cause a current flow in theopposite surface. In general, the greater the component perpendicular tothe direction of induced current, the more effective is the productionof a current in the opposite surface. The manner in which thesephenomena are utilized for the transfer of energy will be described morefully in connection With a practical embodiment of this invention.

In Fig. 2 is shown such a practical embodiment in the form of agenerator of oscillations in the 2400 to 3400 megacycle per-.second bandof the frequency spectrum. This generator, comprising twolighthouse-type triode vacuum tubes particularly designed for highfrequency work and a plurality of concentric cylindrical members asresonant sections, operates in a manner described in my co-pendingapplication entitled Ultra- High Frequency Oscillator, Serial No.624,618, led October 25, 1945. The two tubes, one of which is indicatedat 20, are generally disposed in face-to-face relationship` at oppositeends of the oscillator and are separated by the cylindrical memberswhich constitute the resonant section. Furthermore, the tubes operatewith a phase difference of 180 degrees so that the generator functionsin a manner somewhat analogous to the manner in which a conventionalpush-pull tuned-plate tuned-cathode low frequency oscillator functions.While this analogy islimited by the inadequacy of ordinary circuittheory at extremely high frequencies, it does extend to the requirementfor an energy transfer from the plate-grid resonant section to thecathode-grid resonant section in order toprovide the feedback couplingto support self-oscillation.

In Fig. 2 a section has been cut away to expose the disposition of thecomponents comprising one end of the oscillator. The unexposed end isessentially identical and includes the other triode tube with its anodecap facing that ofthe visible tube. In general, telescopic cylinderrepresents the inner conductor and telescopic cylinderV 6 the outerconductor of a three half Wavelength resonant concentric transmissionline. Thisline constitutes the plategrid resonant section of theoscillator and in the mode in which the generator isoperated-ischaracterizedY by the location of a voltage node and currentV loop atthe generator midpoint defined by a plane 3 3` through the generatortransverseA to its longitudinal axis. Likewise, and'again in general,telescopic cylinder- 6 represents the inner conductor and telescopiccylinder'l the outer conductorof a portion of a ve halfwavelengthresonant concentric transmission line. This line constitutes 4thecathode-grid resonant section and is likewise characterized by thelocation of a voltage node and current loop at the generator midpoint.Since oscillationV requires that anode and cathode voltages withrespectto grid-be essentially in phase, the difference inelectrica-llength of an odd full wavelength between the two sectionsestablishes a phase difference of essentially 180 degrees for'thecurrents' on the outer and inner surfaces of cylinder 6 4at the midpointof the generator. There is no coupling between the tWo resonant sectionsmerely by virtue of their common use of cylinder 6 since, at thefrequencies under consideration, current flow is substantially a surfacephenomenon and the thickness of cylinder 6 is very large in comparisonto the depth of current penetration. The resonant transmission-linesections operate in the usual TEM mode of coaxial transmission lines sothat the electrical field is radial, the magnetic field iscircumferential, and current ows in a direction parallel to thelongitudinal axis of the generator.

Telescopic cylinder 5 comprises in part a central element 5', telescopiccylinder 6 a central element 5', and telescopic cylinder 'I a centralelement l', all of such central elements being held in a fixed positionrelative to chassis 8. The movable elements (5, E, and 1) of the threecylinders provide the means by which the physical lengths of theresonant sections may be changed and thus the generator tuned to adesired frequency within its band of operation.

At the generator midpoint there is cut into centralelement 6 acircumferential slot corresponding to aperture 4 of Fig. 1. This slot,which has been given the reference numeral 9, is best illustrated inFig. 4 which is a diagram of the mid-section of element 6'. It will beseen that this slot is disposed with its length transverse to thedirection of current now along the inner andV outer surfaces of element6 and that it has been provided with a pair of shoulders Il and II not.present in the case of aperture 4 of Fig. l.

Shoulders I0 and II comprise a circular conducting disc xed to element 6across aperture 9 which disc has hadits midsection removed to coincidewith the interception of the aperture and represents a part of the meansby. which, according to the teachings of this invention, the electricallength of a slot of fixed physical length, such as slot 9, is maintainedat an optimum value for the particular frequency tof which thecgeneratormay bectuned In the same Wayin which a lumped capacitance across themidpoint of a half-wave Lecher frame shorted at both ends can ibeutilized to maintain the electrical length while a reduction is made inits physical length, the lumped capacitance across the midpoint of slot9 serves to extend the equivalent electrical length,

capacitance across the slotatits midpointr con-A sists, not only of the,fixed capacitance between shoulders I0 and II, Ibut also of theseriessum of the capacitance between Ashoulder I0 andthe portion ofplate I2immediatelyadjacent thereto and;

the capacitance .between shoulder I I and thepor,- tion of plate I2adjacent to it. Thus, movement of plate I2 may be utilized to vary thistotal capacitance and thereby to vary the electrical length of the slotto the optimum value for the transfer of energy between the tWo resonantsections. Threaded adjusting member I3 is sup` ported from outercylinder element I by meansl of boss I4. Plate I2 is preferably surfacedwith mica to prevent the short-circuiting of shoulders.

I0 and II when plate I2 is adjusted to a position providing littleseparation.

Since slot Sis disposed in a transverse sense to the longitudinal flowof alternating current along the inner surface of element 6", acondition exists, such as wasA described in connection.

with Fig. l, in which slot 9 constitutes a common impedance between theplate-grid resonant section and the cathodegrid resonant section andprovides for coupling by virtue of the mutual impedance which exists.Thus, electromagnetic en-A ergy existing in the Iplate-grid resonantsection surface of elementA 6 at the midpoint tending to be everywhereparallel to the longitudinal axis of the oscillator, and the flow ofthis current in the vicinity of the slot is impeded by a total impedanceconsisting of the slot impedance in shunt with the effective impedanceof the cathode-grid resonant section as seen at the slot looking fromthe plate-grid resonant section into the cathode-grid section.Therefore, if, for example, optimum tuning of the cathode-grid resonantsection is represented by the resonance condition, the effectiveimpedance of the cathode-grid section as measured from terminals at theslot is essentially resistive, and if the slot is tuned to resonance,that is, tuned to represent resistive impedance when taken by itself,the total impedance across the slot to current flowing on the innersurface of element 6 in the vicinity of the slot is resistive and has amagnitude equal to the parallel sum of the slot impedance and thecathode-grid section effective impedance.

In this case, at points corresponding to the oscillator midpoint, thedirections of the current ows on the outer and inner surfaces of element6' corresponding to excitation of the plate-grid and cathode-gridsections each as a whole are op posite at al1 times during the cycle.Also, in this case, the current now around the ends of the slot,corresponding to the excitation of the slot itself, occurs with aquarter cycle phase difference with respect to the current flow in theresonant sections as a whole. The degree of coupling provided by theslot is dependent in part upon the proportion of the circumference ofelement 6' occupied by the slot, and varies from negligible values forslots of innitesimal length to large values where the slot length ismade to approach the circumference of element 6'. For the same tuningconditions with respect to resonance, increasing the slot lengthdecreases the eifective impedances of the plate-grid and cathode-gridresonant sections as measured from opposite points across the slot. Ifthe most desirable phase of excitation of the cathode-grid section withrespect to the plate-grid section differs somewhat from thatcorresponding to direct opposition of the current fiows on the outer'and inner surfaces of element 6', the slot may be detuned fromresonance byzan amount and in a sense such as to obtain a reactive slotimpedance v whichin lcombination with the effective impedances oftheAplate-grid and cathode-grid sections yields the desired phase ofexcitation in accordance with the principles underlying the operation ofthe more familiar circuit networks. gThus, it is seen that therapplication of the teachings of this invention permits the provision ofa slot coupling means affording a desired degree of coupling, andpermitting the adjustment of the phase of excitation to an optimum valueat any frequency in a wide portion of the frequency spectrum.

Although a particular slot shape has been described, this particularityhas been employed, not to establish limits in configuration, but tofacilitate description of the principles of operation. Actually, thedimensions of the aperture and the shape of the aperture may varyaccording to the requirements of a particular situation. The lengthdimension of one-half wave length is a special case, the optimum lengthunder other conditions may be more or less than one-half wavelength. Thesame basic eect is obtainable with apertures in shapes other than simpleslots without exceeding the limits 'of this invention." While referencehas been made herein to a simple along the direction of current flow(slot widthV being taken as negligible). The degree of coupling dependsupon the angle of the slot and varies from zero for the slot parallel tocurrent ow to a maximum value for the transverse position. In thecylindrical generator application,

this feature affords a variable coupling possibility.

separate from the variation ofcoupling possible through changes in theproportion of cylinder circumference occupied. To those versed in theart this obviously represents a convenient means of varying the couplingin certain suitable arrangements such as waveguide apparatusv Again, itshould be understood that, although the coupling slot which has beendescribed is a preferred embodiment of this invention, it by nomeansestablishes the boundary application.v For example, a similar couplingslot, similarly oriented, but inserted in outside cylinder l., at thegenerator midpoint could be used to transfer energy from thecathode-grid resonant section to a waveguide so as to excite thewaveguide in a suitable mode to act as a transmission line between thegenerator and an external load. Further, it will be seen that otherapplications, differing in physical arrangements but the same inprinciples, may be made to advantage. As an example of this, a resonantcircuit may consist of parallel plane conducting walls which may containan aperture or apertures which function to transfer energy in the mannerexplained. For the purposes of this disclosure, such van arrangement isdefined, in the same sense that the plate-grid and cathode-grid sectionsof the generator of Fig. 2 are defined, as a wall enclosed componentpart.

Since certain further changes may be made in the foregoing constructionand different embodi-r factured and used by or for the Government ofvthe United States of America for government purposes without the paymentof any. royalty thereon ortherefor.

What is claimed is:

1. In combination, a plurality of conducting wall members arranged toconstitute a plurality of nested energy containing sections of which atleast two have a common wall member, said two sections having saidcommon wall member being so oriented that the directions of current flowalong the surfaces of said common wall member in said two sections areparallel, a means coupling energy from one of said two sections into theother of said two sections comprising an aperture cut in said commonwall member, and a lumped variable capacitance across said aperture atits physical midpoint, said aperture being, in width, a fractional partof a wavelength at the frequency'of said energy and, in length, lessthan a half wavelength at said frequency, said aperture. also: being'disposed insaidY common wallV memberwith its length dimension.transverse to section removed to coincide with the interception ofsaid. apertureand the other of which is mov- Aable.witl'rrespect`-tosaid xed disc, said nlovability` providing'a range of variable reactancesuch that theA electrical length of said aperture may be made torepresent substantially a half wavelength at said frequency.

2. Inv combination, a plurality of conducting walll members arranged tovconstitute a plurality of. nested energy containing sections of .whichat least two have a common wall member, said two sections having saidcommon wall member being so oriented that the directions of current flowalong the surfaces of said common wallV member in said two'sections areparallel, ameans coupling,

energy from one of said. two sections into the other of said twosections comprising a slot cut in said common wall member transverse tosaid. directions of current flow, said slotl being, in width afractional part of a' wavelength at they frequency of said energy and,in length less thana half Wavelength at said frequency,.a pair ofconduct@V ing parallel plates, one of said plates being xed on saidcommon wallmember at the physical midpoint` f said slot, said plateshaving a width greater thanthat of said slot and a' length less thanthat of said slot, said one plate having its midsection removed to form'an. aperture coincident With but of smaller width than said slot, andmeans for moving said other plateaxially toward and away fromsaid rstplate.l

3. In combination, a plurality of conducting wall members arranged toconstitute a plurality of :nested energy containing sections of which atleast two have a common wall member, said two sectionshaving said commonwall member being so oriented that the directions of current flow alongthe surfaces of said common wall member in said two sections areparallel, a means coupling energy from one of said two sections into theother of said two sections comprising a slot cut in said common wallmember transverse 'to said directions of current flow, said slotbeing,in Width a fractional part of a wavelength atthe frequency of saidenergy and, in length less than a half.

wavelength at said frequency, afpair'of conducting coaxial paralleldiscs', one of said discs being.

fixed on said common wall member `at the physical midpoint of said slot,said discs having a diameter. greater than the slot wid-th and less thanthe.

slot. length, said onev disc having lits midsectionY removed tdformanaperture coincident with but of smaller width .than that. of said slot,and means for moving said other disc axially toward and away from saidone disc.

4. In a cavity type oscillator having concentric cavities forming awallmember common to two cavities on which the current in each of the twocavities is parallelcoupling means between said two cavities providingthe optimum feedback for supporting oscillations comprising, an-

elongated slot in said common Wall member having ay length less than.ahalf Wavelength at the frequency of said. oscillations, said slotbeingl positioned transverse to the flow of said currents, apair ofconducting parallel plates, one` of said plates being iixedI onsaidcommon wall member at the physical midpoint of said slot,.saidplates.

having a width greater than that of said slot and alength less than thatvci? said slot, said oneplate having its midsection removed to form anaperture coincident with but of smaller width than said slot, and meansfor moving said other plate axially toward and away from said iirstplate.

5. In acavity typeoscillator having. concentric cavities forming a wallmember commonto two cavities on which the current ineach of the twocavities is paralleLcoupling means between said two cavities providingthe optimum feedback for supporting oscillations comprising,y anelongated slot in said common wall member having a length less thanahalf wavelength at the frequency ofv said oscillations, said slot beingpositioned Vtransverse to the i-loW of said currents, a pair ofconducting coaxial parallel discs, oneof said discsy being xed on saidcommon Wall member at the physical midpoint of said slot, said discshaving a diameter greater than the. slot Width and lessv thanthe slotlength, said one disc having its midsection removed to form an aperturecoincident with but of smaller width than that of said slot, and meansfor moving said other disc axially toward and away from said one disc.

JOHN E. GIBSON.

REFERENCESv CITED The following references are of record in the le ofthis patent:

UNITED. STATES PATENTS

